A. Field of Invention
This invention generally relates to devices capable of variably attenuating the transmission of electromagnetic radiation using manual and/or automated attenuation means.
B. Description of the Related Art
It is known in the automotive arts to use visible light filters to attenuate the level of light experienced by the occupant of a vehicle. For instance, it is known to incorporate a tinted strip in a top region of a windshield to lessen the glare from the sun. Additionally, it is known to apply tinted films to OEM windows aftermarket, or to incorporate tinted glass into an automobile. It is even known to use suspended particle, electrochromic or similar light attenuation devices to reduce the transmittance of a window.
Electrochromic devices operate by applying a voltage to an electrochromic material, which generally causes an electron transfer to or from a color center. The electron transfer causes the color center to either absorb visible light, thereby resulting in a colored material, or to reduce or eliminate visible absorption, thereby resulting in a colorless, i.e. bleached, state. Generally, a voltage is applied only long enough to cause the absorption or bleaching electron transition. The power can then be turned off and the colored or bleached state remains in a more or less stable condition.
In contrast, one problem that suspended particle devices have is that power must be maintained in order for the device to remain transmissive. When the power is lost, the suspended particles randomize and result in an opaque or translucent material. Therefore, in a de-energized state light attenuation exceeds that which is safe and/or required by government regulation. In fact, if power to the device were lost during operation of the vehicle the operator's vision could be completely obscured, creating a potentially deadly situation. For this reason, electrochromic devices have been preferred over suspended particle or liquid crystal devices.
Some embodiments of the present invention provide an automotive variable attenuated transmission device with a minimum transmittance that meets governmental safety requirements. Furthermore, some embodiments provide a system for manual and/or automatic control of such devices.